This invention relates to the field of holographic articles including microembossed images, such as holograms or diffraction gratings, and, more particularly, to transparent holographic materials and processes for their preparation.
Microembossed holographic and diffraction grating articles or constructions are well known. Typical forms include hot stamping foils, overlays, labels, packaging, stickers and the like. Such articles each contain a layer which can be impressed with a microembossed image. U.S. Pat. Nos. 4,259,285; 5,503,792; 5,164,227 and 5,503,896 describe different methods for microembossing a variety of materials.
For some applications the microembossed layer is subsequently coated with an opaque, reflective layer such as aluminum so that a vivid, intense image is readily visible. Examples of reflective holographic articles are described in U.S. Pat. Nos. 4,945,215 and 5,087,510.
For other applications it is advantageous for the microembossed article to be transparent so that graphics or text may be viewed through the image. To accomplish this effect, it is necessary to apply a further layer that has a refractive index that differs from the refractive index of the microembossed layer by about 0.2 or more. Typically the further layer has a higher refractive index than the microembossed layer and is referred to herein as a HRI coating or layer. Examples of this approach are described in U.S. Pat. Nos. 3,703,407; 4,856,857; 5,351,142; 5,781,316 and 5,948,199. If the further layer has a lower refractive index than the microembossed layer it is referred to herein as a LRI coating or layer.
The HRI or LRI coatings or layers are deposited from the vapor phase to avoid damage to the microembossed layer. Conventional deposition techniques include reactive or non-reactive vacuum vapor deposition, sputtering, electron beam deposition, ion beam assisted deposition and the like. These techniques start with the HRI or LRI material or reactive precursor thereto in solid form, and transfer the HRI or LRI material, in vacuuo, onto the microembossed layer (e.g., see; U.S. Pat. No. 5,464,710). Such techniques are slow and expensive and require bulky, costly equipment that may have no other use.
Accordingly, objects of this invention include providing: 1) solution-borne, HRI or LRI materials which can be used to create transparent holographic articles; 2) preparing transparent holographic articles by means of a process that includes a solution coating step, e.g., traditional printing methods; 3) efficiently preparing low cost transparent holographic articles such as hot stamping foils, overlays, films, labels and 4) other objects and advantages that will be apparent to those skilled in the art.
In one aspect this invention comprises, transparent articles including a microembossed image formed by a process that includes a step of applying solution-borne high or low refractive index material to a microembossed surface using solution or liquid coating method, e.g., a printing technique such as flexography, gravure, offset, intaglio or litho.
In another aspect this invention comprises, liquid-borne HRI or LRI materials which are formulated for application to a microembossed image, e.g., an ink suitable for a conventional printing process.
In yet another aspect this invention comprises, the articles prepared in accordance with the above-described method, which include transparent holographic transfer products such as a hot stamping foil or a security overlay, a transparent holographic adhesive label, a transparent holographic film and the like.
Other advantageous features and objects will become apparent upon reference to the following description of the preferred embodiments when read in light of the attached drawings.